Methods and Devices For Maximizing the Throughput of TCP/IP Data Along Wireless Links

ABSTRACT

The amount of TCP/IP packets which can be sent from an Internet network to a wireless network is maximized by modifying a receive window value of an acknowledgment (ACK) before the ACK is sent on to a source of data packets within the Internet network. The receive window value is modified to take into consideration delay and rate variations which occur in the wireless network.

It is no secret that more and more people wish to receive email andother Internet-based messages on their wireless devices (e.g., cellphones, PDAs). Historically, however, the Internet (known as a so-calledwired network, e.g., a network where signals are sent via a physicalcopper wire instead of over-the-air) and wireless networks developedseparately from one another. As a result, the most common way of sendingand receiving data over the Internet, called Transmission ControlProtocol/Internet Protocol (TCP/IP), is not the most common way thatdata is sent over a wireless network. One of the challenges inconnecting a wired network to a wireless network is related to thedifferent speeds at which these networks operate and their relativereliabilities. Wired networks are more reliable than wireless networksand typically send data at rates which are much higher than the ratesused by wireless networks. When high speed data is sent from a wirednetwork in a reliable manner to a much slower speed, sometimesunreliable wireless network, some kind of “traffic cop” is needed inbetween to ensure proper communications.

One such device is called a radio network controller (RNC). Typically,an RNC contains a buffer (e.g., memory) for storing incoming TCP/IP datafrom an Internet-based network. Subsequently, this data must bere-transmitted by the RNC to a wireless device over a slower speed, lessreliable wireless network. Because the RNC may receive a large amount ofdata over a very short period of time, it may be unable to transmit allof its stored data to a wireless device before becoming congested (i.e.,before the buffer overflows). Once the buffer overflows, packets of datain the buffer are dropped; an undesirable outcome.

Sometimes the wireless link between the RNC and mobile device is lost.This also results in a loss of packets. Realizing this, most existingRNCs retain a copy of each packet until the wireless device sends anacknowledgement (ACK) that it has received the packet successfully. ACKsare also used to regulate the amount of packets sent from a source ofTCP/IP packets within an Internet network to an RNC for eventualdelivery to a wireless device. If the source does not receive an ACK, nonew packets will be sent.

The re-transmission and loss of packets introduces delay and ratevariations. Large delay and rate variations are common in a wirelessnetwork. In contrast, such variations are relatively small withinInternet-based networks as well as other wired networks. In fact, inorder to format data packets according to TCP/IP, most TCP/IPcommunication equipment assumes that there will be little fluctuation inboth the delay and rate at which packets are transmitted and received.These assumptions, however, do not apply when TCP/IP packets originatingat an Internet-based network need to eventually be sent over a wirelessnetwork.

To allow TCP/IP based data to be transmitted between an Internet-basednetwork and a wireless device, there must be some way of adjusting theflow of TCP/IP packets when delay and rate variations fluctuate in orderto maximize the amount of information (i.e., throughput) sent between anInternet-based network and a wireless device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a simplified block diagram of an Internet-based networkconnected to a wireless-based network.

FIG. 2 a depicts a simplified illustration of a congestion window usedby a source of TCP/IP data.

FIG. 2 b depicts a simplified illustration of a receive window includedin the header of an acknowledgement packet according to one embodimentof the present invention.

FIGS. 3 a, 3 b, 4 a, 4 b, 5, and 6 a, 6 b graphically depict throughputversus queue length, latency and loss, respectively.

SUMMARY OF THE INVENTION

The flow of TCP/IP packets to a wireless network or device is maximizedin accordance with the present invention by a modifying a receive windowvalue of an ACK before transmitting the ACK on to a source of packetswithin an Internet or wired network. The receive window is modified totake into consideration delay and rate variations which occur in thewireless network.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, there is shown a simplified view of anInternet-based network 201 operable to send TCP/IP formatted datapackets (“TCP/IP data” for short) from a source 200 to a wireless device300 within wireless network 301 via an RNC 100. It should be understoodthat the source 200 is the source of TCP/IP data which is ultimatelysent to wireless device 300. The RNC 100 and wireless device 300 maybelong to a number of different types of wireless networks, for example,a third generation (3G) wireless network. Though not shown in FIG. 1, itshould be understood that other devices are typically used in betweenRNC 100 and wireless device 300, for example, one or more base stations.For ease of explanation, these base stations have been omitted fromFIG. 1. At some point in time a TCP/IP data message is sent from thesource 200 via pathway 23A to RNC 100. The message is stored in databuffer 102 before being transmitted to wireless device 300 via pathway23B. Upon receipt of each packet within the message, wireless device 300is operable to send an ACK along pathway 32A back to RNC 100. ACKsreceived by RNC 100 are stored in ACK buffer 103. This ACK is sent bythe RNC 100 back to the source 200 via pathway 32B to inform the source200 that a corresponding packet has been received. Upon receiving anACK, the source 200 is typically prompted to transmit an additionalpacket on to RNC 100 provided such a transmission does not violatelimits set by a congestion window contained within source 200. FIG. 2 ashows a simplified illustration of a congestion window 321 within memoryunit 320 which is part of source 200. More specifically, congestionwindow 321 comprises a data rate value (e.g. 32 kilobytes) which acts asan upper limit on the amount of packets which can be sent to RNC 100.

The congestion window value, however, is just an estimate. To realizemaximum throughput, the estimate needs to be adjusted based on the near,real-time conditions of wireless network 301 and buffer 102.

In accordance with one embodiment of the present invention, RNC 100 isoperable to modify an ACK received from the wireless device 300 beforeforwarding it on to the source 200. The modified ACK contains a value(e.g., 64 kilobytes) based on the present condition of wireless network301 which will help the source 200 select the amount of data which canbe sent to the wireless device 300 to maximize throughput withoutoverflowing or underflowing buffer 102. It should be understood that thebuffer 102 shown on FIG. 1 is a per-user buffer. That is, each user orsource of data within network 201 is given an amount of buffer spacewithin RNC 100. This buffer space is referred to as buffer 102. RNC 100is also shown comprising a control section 101. In one embodiment of thepresent invention, the control section 101 is operable to estimate delayand rate variations of wireless links 23B and 32A in real-time, andmodify an ACK before it is forwarded on to the source 200. As is knownby those skilled in the art, delay and rate variations may be derived,for example, by detecting the number of packets within link 23B and thenumber of ACKs within link 32A. Though shown as separate units, itshould be understood that the control section 101 and buffers 102,103may comprise a single unit or may be further broken down into additionalunits. Further, it should be understood that the control section 101and/or buffers 102,103 may be realized in whole or in part in hardware,firmware, software or some combination of the three.

FIG. 2 b depicts a highly simplified illustration of an ACK 420comprising a receive window value 421. In one embodiment of theinvention, the control section 101 is operable to modify the receivewindow value 421 of the ACK 420 based on the estimated delay and ratevariations. For example, the receive window value may be a data rateequal to 32 kilobytes. After modification this rate may change to, forexample, a value between 1 kilobyte and 64 kilobytes. After the receivewindow value 421 is modified, it is sent on to the source 200 embeddedwithin modified ACK 420. Upon receiving the modified receive windowvalue, source 200 is operable to compare the value to a congestionwindow value. Thereafter, source 200 is operable to select the lesser ofthe two in order to ensure that an overflow condition does not occur.The lesser value is used as the transmission rate at which packets aresent to the buffer 102.

As was mentioned above, the aim of the present invention is to maximizethroughput. To do so, at least one packet must always be available to betransmitted from buffer 102 to prevent underflow and there must be nopackets lost due to overflow. Both of these requirements are satisfiedby the use of a modified ACK.

Having provided a general overview of the invention, a more detaileddiscussion will now be provided.

Consider now the arrival of an iTH packet at buffer 102 sent by source200 (e.g., a web server). Let Y_(i) represent the number of packetswithin the wireless link 23B when packet i arrives, where Y_(i) is afunction of the varying rates and delays in the forward and reversedirections of the wireless links 23B, 32A, and let N_(i) (≦B) be thenumber of packets in buffer 102 when packet i arrives, where B is thesize of buffer 102. Assuming that delays associated with wired network201 are insignificant as compared with wireless network 301, all packetsfrom source 200 are stored in buffer 102 and all ACKs are immediatelysent to source 200 leaving no outstanding packets in the wired network201 which need to be sent to wireless device 300. Under theseconditions, the sum of data packets in the buffer 102 and in thewireless link equals a receive window size, W, or

W=N _(i) +Y _(i)+1.  (1)

To avoid buffer underflows, the window size (i.e., data rate value), W,must obey:

∀iY _(i)+1<W (no underflow).  (2)

Equation (2) states that the receive window size must be greater thanthe instantaneous number of packets in the wireless link, Y_(i), andthere must be at least one packet in buffer 102 for there to be nounderflow. However, because we also assume that Equation (1) is true,N_(i)≦1 when there is no underflow and thus Equation (2) is also asufficient condition for no underflow.

For there to be no overflow (dropped packets) from Equation (1), B mustbe ≧W_(i)−Y_(i). In other words,

∀iY _(i) +B≧W _(i) (no overflow).  (3)

In the discussion above, it was assumed that every packet wasacknowledged with a separate ACK. However, RNC 100 is further operableto vary the value of the receive window size accordingly whenacknowledgements represent more than one packet.

As described previously above, RNC 100 is operable to generate a receivewindow size value. Because this value changes when the delay and ratevariations of network 301 changes (represented by links 23B, 32A) innear real time, this value can be referred to as a variable, dynamicreceive window value (“WRD” for short).

Equation (4) below summarizes the steps involved in generating thisvalue, where Y is a current estimate of the number of packets withinwireless links 23B, 32A, without violating Equations (2) and (3):

On Deque of each Data packet (4) Y = Y + 1 On Enque of eachAcknowledgement Y = Y + 1 Set W^(r) = Y + B Transmit the Acknowledgementto the source.

This technique is conservative as it satisfies Equation (3) (nooverflow) but uses a larger receive window size value as compared withexisting techniques. Because the present invention involves modifyingthe value of the receive window, the throughput (W/RTT) provided by thepresent invention is as good or better than the throughput of existingtechniques because:

${{{If}\mspace{14mu} \frac{W}{RTT}} \leq R},{{{then}\mspace{14mu} \frac{W}{RTT}} \leq {\frac{W + k}{{RTT} + {k/R}}\mspace{11mu} {\forall{k \geq 0}}}}$

where W is the window size, RTT is the round trip time and R is theaverage rate of the connection.

If the window size of an existing technique (e.g., WindowRegulator—Static, “WRS”) is B, the size of the present window is givenby B+Y, where Y is always a positive number.

Though the WRD technique substantially prevents overflow, it may notprevent underflow. For example, suppose sometime after the comparisondiscussed above occurs and a transmission rate is set, but before a nextmodified ACK is received by source 200, the condition of wirelessnetwork 301 changes. If the change makes it possible to send morepackets to device 300 yet the transmission rate is set too small, thebuffer 102 will underflow because all packets sent from source 200 toRNC 100 will be re-transmitted to device 300 immediately, thus leavingbuffer 102 empty, before a next ACK is received by source 200. Withoutthe reception of a modified ACK by source 200, no new packets will besent from source 200 to RNC 100 to fill up buffer 102.

In a further embodiment of the invention, RNC 100 is operable to storeACKs and forward them to source 200 only when needed to prevent theunderflow of buffer 102. For example, at any given point in time thedelay or rate variations within wireless network 301 may remain fairlyconstant. If this occurs, it is arguably unnecessary for RNC 100 tocontinue to modify an ACK in the same manner and forward it on to source200. Instead, RNC 100 may be operable to store one or more ACKs untilsuch time as control unit 101 detects buffer 102 is approaching asubstantially empty state and/or the delay and rate variations ofwireless network 301 substantially change, at which time RNC 100 isoperable to modify a receive window value of one or more ACKs andforward the modified ACKs on to source 200. In the former case amodified receive window value of an ACK is sent to prompt the source 200to send packets to buffer 102 to prevent underflow. In the latter case amodified receive window value of an ACK is utilized to adjust packetthroughput as discussed in detail before and above.

This additional embodiment can be referred to as a WindowRegulator-Buffer or “WRB”.

Equation (5) summarizes the operation of RNC 100 operating using WRB:

On Deque of each Data packet (5) Y = Y + 1 If there is anAcknowledgement stored in the Acknowledgement buffer, then W = Y + B +B_(a) Transmit Acknowledgement to the source endif On Enque of eachAcknowledgement, set Y = Y − 1 W = Y + B + B_(a) If (new AcknowledgementAND (W < previous value of W)) Store Acknowledgement in theAcknowledgement Buffer B_(a) = B_(a) + 1 else transmit Acknowledgementto the source endif.where B_(a) is the size of the acknowledgement buffer.

Note that in WRB, B_(a) can only be increased until it converges to somevalue Y_(max) and ∀iW_(i+1)≧W_(i). Note also that Equation (2) issatisfied because ∀iY_(max)≧Y_(i) and Equation (3) is also satisfiedbecause the delay from wired network 201 is insignificant.

The queue utilization, Q_(WRB), is given by:

$\begin{matrix}{Q_{WRB} = {\frac{1}{k}{\sum\limits_{i = 1}^{K}{1{\left\{ {{Y_{i} + B + B_{a}} > Y_{i + 1}} \right\}.}}}}} & (6)\end{matrix}$

If the size of the acknowledgement buffer is not limited (becauseacknowledgements consume very little memory and do not impact thelatency of new flows), then B_(a) will grow large enough to absorb themaximum variation on the wireless link. Therefore,Pr(Y_(i)+B+B_(a)>Y_(i+1)) approaches one. Thus, if the flow lasts longenough, WRB achieves the maximum utilization of 1. The queue utilizationQ_(WRB) is

$\begin{matrix}{Q_{WRB} = {{\frac{1}{k}{\sum\limits_{i = 1}^{k}1}} = 1.}} & (7)\end{matrix}$

FIGS. 3 a, 3 b through 6 a, 6 b graphically summarize the results ofthroughput v. queue length (single user), throughput v. queue length(multiple users), throughput v. round trip wired latency and throughputv. loss, respectively of existing techniques and techniques provided bythe present invention.

Summarizing the results shown in these figures, it can be said thatbecause drop-tail (DT) and WRS cannot adapt to large rate and delayvariations in a wireless channel, TCP throughput suffers.

Though an existing technique known as “acknowledgement regulator” (AR)adapts reasonably well to large variations, it does not perform wellwhen wired latencies are significant because its associated estimationerrors cause a degradation in TCP throughput.

In contrast, WRD techniques perform well in terms of throughput androbustness when faced with reasonable wired latencies. However, WRD doesnot perform as well as WRB when losses occur due to a congested buffer.

WRB performs the best in terms of maximum throughput, robustness in theface of relatively large wired network latencies and in the face ofrelatively large losses of packets due to congested buffers.

1. (canceled)
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 5. A method formaximizing the throughput of TCP/IP data comprising the steps of:determining whether a data buffer is substantially close to empty;determining whether delay and rate variances have substantially changed;and storing one or more ACKs when said buffer is not substantially closeto empty or when said variances have not substantially changed.
 6. Themethod as in claim 5 further comprising modifying a receive window valueof an ACK packet when said buffer is substantially close to empty orwhen said variances have substantially changed; and transmitting amodified ACK packet containing the modified receive window value to asource of TCP/IP data when said buffer is substantially close to emptyor when said variances have substantially changed.
 7. (canceled) 8.(canceled)
 9. (canceled)
 10. (canceled)
 11. A device for maximizing thethroughput of TCP/IP data operable to: determine whether a data bufferis substantially close to empty; determine whether delay and ratevariances have substantially changed; and store one or more ACKs whensaid buffer is not substantially close to empty or when said varianceshave not substantially changed.
 12. The device as in claim 11 furtheroperable to: modify a receive window value of an ACK packet when saidbuffer is substantially close to empty or when said variances havesubstantially changed; and transmit a modified ACK packet containing themodified receive window value to a source of TCP/IP data when saidbuffer is substantially close to empty or when said variances havesubstantially changed.